Soft-bodied invertebrates, such as leeches, worms, and slugs, have successfully colonized marine, terrestrial, and fossorial (underground) environments. They do so with complex structures that can rapidly change shape on command. Some of these animals contain a central fluid-filled cavity. Contraction of a muscle component of the cavity induces an expansion of other parts of the cavity and of its surrounding muscle. Animals with these body architectures have a hydrostatic skeleton. However, other soft structures, such as tongues, trunks, or tentacles, have higher power-to-mass ratios. These structures consist solely of muscle fibers with no central fluid-filled cavity and have been termed muscular hydrostats. By deploying muscle groups arranged in ordered configurations—longitudinally, circumferentially, or helically—these structures are capable of both rapid and dexterous movements. The skins of soft-bodied animals have many sensors embedded within them. Their nervous systems coordinate their many degrees of freedom in order to locomote in a variety of ways, including peristaltic crawling, anchor-and-extend, and swimming.
It is often desirable to remotely access the interior of structures, which are difficult or unsafe for a human to directly enter, possibly because of constrained and/or labyrinthine physical conditions. For example, inspections or repairs may be required by pipelines and plumbing systems of all types, search and rescue missions may need to look for disaster victims within collapsed buildings, or scientists may wish to explore an underwater cave system. In addition, there may be applications in the medical field for robots, which can travel through the human body, such as through the vasculature or the digestive system, to observe and manipulate the patient's internal structure less invasively than currently available methods for medical procedures such as endoscopy or angioplasty.
Robots with locomotion capabilities similar to those of soft-bodied animals would be able to complete many useful tasks, including conducting reconnaissance through small crevices, exploring complex terrain for search and rescue missions, actively pushing an endoscope throughout the entire gastrointestinal (GI) tract, or performing minimally invasive surgery. Currently, remote-controlled robots are in limited use for such applications, but the propulsion systems and other technical aspects of these robots are not well-suited for widespread use. For example, serpentine robots have had the most success in some of these areas to date, but rely on a motion that does not work as well in the most confined spaces where burrowing is required.